b43legacy: Fix assigning negative value to unsigned variable

fix warning reported by smatch:
drivers/net/wireless/broadcom/b43legacy/phy.c:1181 b43legacy_phy_lo_b_measure()
warn: assigning (-772) to unsigned variable 'fval'

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/1648203433-8736-1-git-send-email-baihaowen@meizu.com